Mass rename SourceLoc and Tag (#860)

This does a mass rename of:

-   `SourceLoc()` -> `source_loc()` for property naming
    - `loc` -> `source_loc_` for underscore+consistency
    - Generally changing function args to `source_loc` for consistency
-   `Tag()` -> `kind()` for property naming and `Kind` parity
    - `tag` -> `kind_` for underscore

Also renames `Pos` and `Results` on `Action`. These are a bit of an exception in that most base classes only have `Tag` and maybe `SourceLoc`, whereas `Action` has a little more. I felt okay having `source_loc()` and `kind()` on the base class where children do `Exp()` and the like, but it felt weird to me to mix it on the same class.

The reason for doing this cross-class in one PR is so that I can do it efficiently with a global replace in the codebase, rather than e.g. changing `Expression` but having to read through compiler errors to determine where it's calling `Expression`'s `Tag` versus a different `Tag`. The end result should be equivalent.

executable_semantics/interpreter/action.cpp

@@ -22,7 +22,7 @@ namespace Carbon {
 using llvm::cast;
 
 void Action::Print(llvm::raw_ostream& out) const {
-  switch (Tag()) {
+  switch (kind()) {
     case Action::Kind::LValAction:
       out << *cast<LValAction>(*this).Exp();
       break;
@@ -36,11 +36,11 @@ void Action::Print(llvm::raw_ostream& out) const {
       cast<StatementAction>(*this).Stmt()->PrintDepth(1, out);
       break;
   }
-  out << "<" << pos << ">";
-  if (results.size() > 0) {
+  out << "<" << pos_ << ">";
+  if (results_.size() > 0) {
     out << "(";
     llvm::ListSeparator sep;
-    for (auto& result : results) {
+    for (auto& result : results_) {
       out << sep << *result;
     }
     out << ")";